Many ion optical devices can only function well with ions having energies within a limited energy range. Examples include electrostatic lenses, in which chromatic aberrations cause defocusing, RF multipoles or quadrupole mass filters in which the number of RF cycles experienced by the ions as they travel the finite length of the device is a function of the ion energy, and magnetic optics which disperse in both mass and energy. Reflectors are typically designed to provide energy focusing so as to compensate for a range of ion beam energies, but higher order energy aberrations usually exist.
Tandem mass spectrometry is a well known technique by which trace analysis and structural elucidation of samples may be carried out. In a first step, parent ions are mass analysed/filtered to select ions of a mass to change ratio of interest, and in a second step these ions are fragmented by, for example, collision with a gas such as argon. The resultant fragment ions are then mass analysed usually by producing a mass spectrum.
Successful operation of the fragmentation device that carries out fragmentation of parent ions is likewise limited by the degree of energy spread of ions that enter it. For example, an energy spread in excess of about 10-20 eV affects fragmentation: higher energies result in only low-mass fragments, while lower energies result in little fragmentation.